Intracavity absorption

Sensitivity can be improved by factors of 10 using intracavity absorption, placing an absorber inside a laser resonator cavity and detecting dips in the laser emission spectmm. The enhancement results from both the increased effective path length, and selective quenching of laser modes that suffer losses by being in resonance with an absorption feature. 

Goldman, A. et al.. Fiber laser intracavity absorption spectroscopy of ammonia and hydrogen cyanide in low pressure hydrocarbon flames, Chem. Phys. Lett., 423, 147, 2006. 

A number of other laser spectroscopic techniques are of interest but space does not permit their discussion. A few specialized methods of detecting laser absorption worthy of mention include multiphoton ionization/mass spectrometry (28), which is extremely sensitive as well as mass selective for gas-phase systems optically detected magnetic resonance (29) laser intracavity absorption, which can be extremely sensitive and is applicable to gases or solutions (30) thermal blooming, which is also applicable to very weak absorbances in gases or liquids (31) and... 

E. Unger and G. Patonay, Near-infrared laser diode intracavity absorption specttometry, Anal. Chem. 61, 1425-1427 (1989). 

The last method has been pushed to an impressive sensitivity by putting the probe inside the cavity of a cw dye laser oscillating on several modes close above threshold. The sensitivity of such a broad-band dye laser to selective intracavity absorption on a single mode is proportional to the number of oscillating modes due to... 

One method which employs the saturable absorption of intracavity gaseous absorbers has turned out to be strikingly successful333) As explained in the last section, the absorption profile of a gas interacting with a monochromatic standing wave inside the laser cavity exhibits a sharp minimum at the center of the unsaturated ab-... 

We have studied first the OH radical in a low pressure flame (15 torr < p < 80 torr) to obtain quenching times longer than our pulse duration of 4 ns. This exciting radiation is derived from the second harmonic of a dye laser pumped by a nitrogen laser. An intracavity Fabry-Perot etalon assures a laser spectral width (AX = 2 10-12 m) closely matching the absorption line. We have excited the Qi7 line (X = 308.9734 nm) of the 2 +(v =0) - 2II(v"=0) transition of OH. 

Intracavity dye laser spectroscopy (IDLS) can be a powerful technique for detecting trace species important in combustion. The technique is based on the phenomenal sensitivity of a laser to small optical losses within the laser cavity. Since molecular absorptions represent wavelength-dependent optical losses, the technique allows detection of minute quantities of free radicals by placing them inside the laser cavity and monitoring their effect on the spectral output of the laser. 

Intracavity enhancement, relative to conventional single pass absorption spectroscopy, is due to mode competition and to threshold effects. A simple calculation of the latter for a single mode laser, starting with... 

Intracavity absorption by I- vapor has been studied for a cw dye laser. The sensitivity enhancement varies from l(r at pump powers near threshold (550 mW and 790 mW) to about 500 at the highest pump powers (near 5 watts). The results can be interpreted quantitatively in terms of a previously proposed theory. 

Several species which are weak absorbers or which may not fluoresce readily, may be detected using intracavity laser absorption. This is a much more sensitive method of detection than the conventional absorption measurements because the output power of a laser is very sensitive to small variations in its gain which, in term, depends on the concentration of absorbing species within the cavity. Vibrational state populations for the CO produced in the reaction... 

Direct spectroscopic measurements of absorptions could provide substantial and much-needed complimentary information on the properties of BLMs. Difficulties of spectroscopic techniques lie in the extreme thinness of the BLM absorbances of relatively few molecules need to be determined. We have overcome this difficulty by Intracavity Laser Absorption Spectroscopic (ICLAS) measurements. Absorbances in ICLAS are determined as intracavity optical losses (2JI). Sensitivity enhancements originate in the multipass, threshold and mode competition effects. Enhancement factor as high as 106 has be en reported for species whose absorbances are narrow compared to spectral profile of the laser ( 10). The enhancement factor for broad-band absorbers, used in our work, is much smaller. Thus, for BLM-incorporated chlorophyll-a, we observed an enhancement factor of 10 and reported sensitivities for absorbances in the order of lO- (24). 

Figure 10 shows the schematics of the experimental setup used for intracavity laser absorption spectroscopy (ICLAS) of bilayer lipid membranes (BLMs). Simultaneous electrical and ICLAS measurements were carried out in a two-compartment container constructed from two 1 cm path lengths quartz cells (Figure 11). 

Figure 10. Schematics of the experimental setup for intracavity laser absorption spectroscopy (ICLAS). CD chopper driver PM power meter Mj, M2, M3, M4 spherical high reflection mirrors Mp = pump mirror MN monochromator PMT photomultiplier SP silicon photocell PC Pockels cell WF wedged filter LIA lock-in amplifier R recorder MS microscope OF optical fiber S sample (solution on BLM) IEM instruments for electrical measurements (see Figure 2).

It is possible to measure the quantum yield of the radical process (eq. 22) by the difference in CO yields with and without NO present. Clark and co-workers (54,55) have used this technique to measure the yield of radical process 22. They find a limiting value of at high NO pressures of 0.70, in good agreement with the values of 0.68 of Lewis and Lee (143) and 0.76 of Horowitz and Calvert (115). The obvious explanation for the increase in CO yield is that complete scavenging of HCO by NO allows an extra CO molecule to be generated. Radical reaction kinetics involving HCO and O2 (and NO) have been recently studied by flash photolysis (215) and laser photolysis/intracavity absorption (56). 

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